1. Field of the Invention
This invention relates to hydraulic wheel brake systems used on motor vehicles such as automobiles, trucks, motorcycles, buses and aircraft. More particularly, the invention relates to methods and apparatus for reducing the duration of lockup of a vehicle's wheels during hard braking or braking on slippery surfaces, thereby reducing the tendency of the vehicle to go into an uncontrolled skid and increasing the effectiveness of the vehicle's braking system.
2. Description of Background Art
All conventional motor vehicles, including automobiles, trucks, motorcycles, buses and aircraft have brakes attached to wheels of the vehicle to permit the vehicle to be slowed or stopped as required. With the exception of the air brake systems used by some buses and trucks, most motor vehicles employ hydraulic brakes.
Whichever braking system is used, all motor vehicles having multi-wheel braking systems are likely to sometimes encounter conditions in which one or more of the wheels to which a braking force is applied "lockup," i.e., cease to rotate. Such wheel lockups can cause the vehicle to skid in an uncontrolled fashion.
Wheel lockup results when braking torque applied to a wheel is sufficient to cause the surface of the wheel in contact with the roadway to exert a tangential frictional force exceeding the breakaway force threshold at which the wheel tends to slide rather than roll on the surface. This breakaway force depends on a number of factors, but is dependent primarily on the coefficient of sliding friction between the wheel and the roadway. Thus, when a vehicle is on a wet or icy roadway, the coefficient of sliding friction between the circumferential surface of the wheel and the roadway may be quite low. Under these conditions, the wheels of the vehicle will lockup even with the application of relatively modest braking forces.
Wheel lockup lasting for an uncontrolled duration is undesirable, since it can cause a vehicle to move in directions not controllable by the driver. Thus, for example, if one or more wheels on one side of a vehicle lockup while the other wheels continue to rotate, the vehicle path may skid, deviating from the path which the driver desires the vehicle to take. In some skids caused by wheel lockup, the vehicle may spin around 360 degrees or more. Obviously, such uncontrolled skids caused by wheel lockup can be very dangerous if not life threatening, and should be avoided.
Wheel lockup does not always result in an uncontrolled skid, yet is still undesirable. Thus, wheel lockup can reduce braking efficiency and increase stopping distance, for the following reason.
When a wheel is rolling with respect to a road surface, and brakes are applied, reduction of the tangential velocity of the circumferential surface of the tire relative to the linear velocity of the vehicle with respect to the road produces a frictional force between the tires and road which is responsible for slowing the vehicle. The magnitude of this frictional force, and therefore of the rate of vehicle deceleration, is proportional to the coefficient of friction between the tire and road. To maximize the possible braking deceleration, then, the coefficient of friction between tire and roadway should be maximized.
If a tire is rolling with respect to the road, the coefficient of friction which determines maximum braking force is the static coefficient of friction, since the tire surface is not sliding on the roadway. However, when wheel lockup occurs, the tire surface slides with respect to the roadway. The frictional braking force is in this case determined by the kinetic coefficient of friction between the tire and road. Since the kinetic coefficient of friction is smaller than the static coefficient of friction, maximum braking force achievable with wheels locked up is substantially smaller than that obtainable with the wheels rotating. Hence, uncontrolled wheel lockup results in increased stopping times and distances, and should therefore be avoided.
In recent years, a significant amount of research and engineering effort has resulted in the development and installation on motor vehicles of braking systems intended to minimize the occurrence of uncontrolled wheel lockup and accompanying uncontrolled skids and reduction in braking effectiveness. Such systems are often referred to as ABS systems (anti-skid or anti-lock braking systems). Typically, existing ABS are relatively complex and costly. Their complexity and cost stems in part from the fact that most existing ABS systems use individual wheel lockup sensors on each wheel, a computerized control system responsive to input signals produced by the wheel sensors, and valves responsive to command output signals produced by the control system in applying the proper amount of hydraulic pressure to the slave cylinder on each wheel brake to maximize braking force while avoiding wheel lockup. In addition to the cost and complexity of existing ABS systems, they are, for the most part, ill-suited to retrofitting installation on existing motor vehicles.
Prior art references disclosing systems for minimizing wheel lockup include the following U.S. patents:
U.S. Pat. No. 4,109,970, Ashby, Jr., Aug. 29, 1978, Anti-Skid Brake Control System. PA1 U.S. Pat. No. 4,113,323, Haney, Sept. 12, 1978, Anti-Skid Power Valve. PA1 U.S. Pat. No. 4,715,665, Ostwald, Dec. 29, 1987, Brake Slip Controlled Brake For Automotive Vehicles.
The automatic wheel lockup control apparatus according to the present invention was conceived of to provide an alternate lockup control system which may be easily installed during the manufacture of a motor vehicle, or be retrofitted into existing vehicles. Its primary application would be in vehicles having a dual braking system, although installation on a single braking system will convert the latter to a dual system. Most modern vehicles have a dual braking system in which one hydraulic braking circuit drives the front wheel brakes, or diagonally opposed front and rear wheel brakes. The other circuit drives the real wheels, or the other diagonally opposed set of wheels. In such a system, damage to one hydraulic circuit will not disable the other circuit, ensuring the operability of at least the braking system for two of the four wheels.